N-(phosphonomethyl)glycine (known in the agricultural chemical art as glyphosate) and its salts are conveniently applied as a post-emergent herbicide in aqueous formulations. These compositions are highly effective and commercially important broad-spectrum herbicides useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
One of the more widely accepted methods of making N-(phosphonomethyl)glycine compounds comprises the liquid phase oxidative cleavage of a carboxymethyl substituent from an N-(phosphonomethyl)iminodiacetic acid (“PMIDA”) substrate using an oxygen-containing gas in the presence of a heterogenous oxidation catalyst. For example, N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid (“PMIDA”) with oxygen in accordance with the following reaction sequence:

Processes for the preparation of N-(phosphonomethyl)iminodiacetic acid are well known in the art. For example, U.S. Pat. Nos. 4,724,103 and 4,775,498 to Gentilcore, incorporated by reference herein, describe a commercially useful process for making PMIDA from an aqueous solution of an alkali metal salt of iminodiacetic acid (e.g., the disodium salt or “DSIDA”), a strong mineral acid, a source of phosphorous acid and a source of formaldehyde. In a first step, a strong acid salt of iminodiacetic acid (“IDA”) is formed by reacting the DSIDA with aqueous HCl. In a subsequent phosphonomethylation (“PM”) step, the IDA acid salt is reacted with phosphorous acid and formaldehyde to form PMIDA. PMIDA concentration in the reaction solution increases as the reaction proceeds and eventually a highly supersaturated solution is formed. Before formaldehyde addition is complete, a PMIDA supersaturation critical point is reached resulting in a crystallization event that is sudden, violent and nearly instantaneous thereby causing PMIDA to “crash” from solution. Formaldehyde addition and PMIDA generation continues after initial crystallization in a dynamic system of concurrent PMIDA generation and crystallization. Reaction by-products and impurities include, for example, N,N-bis-(phosphonomethyl)glycine (“glyphosine”), glycine, N-methyl-N-(phosphonomethyl)glycine (“NMG”), N-methyl-iminodiacetic acid (“N-methyl IDA”), iminodiacetic acid, unreacted formaldehyde, and the sodium-acid anion salt (e.g., NaCl). Gentilcore teaches that the PMIDA process may be optimized by splitting the total DSIDA charge between the hydrolysis and PM steps thereby converting only that portion of DSIDA added to the hydrolysis reactor to the IDA acid salt. The remainder of the DSIDA is added along with the formaldehyde to the PM reactor. The DSIDA added during the PM step is converted to IDA acid salt in situ by HCl released upon consumption of IDA.HCL in the phosphonomethylation reaction and any free HCl remaining in solution thereby minimizing the amount of acid that it is necessary in either the hydrolysis or the phosphonomethylation steps to prevent generating significant or excessive amounts of N-methyl-iminodiacetic acid by-product. By using HCl more efficiently, by-product containing less sodium chloride (NaCl) is generated, thereby producing a purer product and requiring less water to solubilize the NaCl.
U.S. Pat. No. 5,688,994 to Baysdon et al., also incorporated by reference herein, describes a process for preparing PMIDA from a source of IDA wherein the formaldehyde source and source of phosphorous acid are simultaneously infused into the reaction mixture. PMIDA is precipitated from the reaction mixture by cooling or pH adjustment. Disadvantageously, crystallization by cooling results in protracted processing time with an associated inefficient use of process equipment at the expense of decreased throughput.
The prior art processes are complicated by the necessity of removing the sodium-acid anion salt from the PMIDA product. Where the acid is HCl and the salt is NaCl, a significant portion of the NaCl is present in the PMIDA product slurry as a solid. Sodium chloride has low solubility in the presence of HCl due to the common ion effect, while PMIDA is readily soluble in the presence of HCl. By contrast, PMIDA has a low solubility in water under neutral conditions, while NaCl is readily soluble. Thus, salt removal requires that the NaCl be dissolved in the reaction solution after the formation of PMIDA is complete. Dissolution is done by adding a dilute base, such as sodium hydroxide, to the reaction mixture so that the pH is adjusted to the point at which NaCl is soluble. Adding additional water ensures that most of the NaCl is solubilized. The pH at which NaCl is readily soluble generally corresponds to the PMIDA isoelectric point, i.e., the point of minimum PMIDA solubility. Thus, upon pH adjustment conditions are most favorable for PMIDA crystallization. Crystallized PMIDA can then be isolated from the mixture by solid-liquid separation means known to those of skill in the art such as, for example, filtration or centrifugation. Isolated PMIDA may then be washed to remove residual NaCl and other impurities, and dried.
PMIDA crystallization in processes known in the art may be generalized as follows. First, rapid crashing of PMIDA from solution results in a large number of relatively small crystals that tend to form dense packed beds in solid liquid separation equipment thereby exhibiting low permeability and liquid removal capability resulting in protracted processing times and associated process bottlenecking and incomplete impurity removal. Second, rapid crystallization may increase occluded (i.e., included or “trapped”) impurity rich mother liquor in the crystalline structure, which is difficult to remove in the PMIDA isolation and washing steps. Third, the relatively large volume of water required to solubilize NaCl may lead to an increased loss of PMIDA in the mother liquor and also result in waste volume increase. Moreover, the large volume further contributes to solid-liquid separation equipment bottlenecking issues.
Generally, a high degree of supersaturation driving force is necessary to initiate crystal nucleation. In various crystallization systems, seeding of saturated solutions is known for initiating crystallization, and in both saturated and non-saturated systems seeding may be used to provide sites for accretion of crystals and to regulate crystal growth. For example, where product is to be crystallized from a reaction solution, the solution may typically be cooled or its pH adjusted to establish supersaturation and, thus, generate a driving force for crystallization. In various known systems, seed crystals may be added prior or subsequent to the adjustment which establishes supersaturation. See, e.g., Liaw U.S. Pat. No. 5,047,088. One of skill in the art will recognize that seed crystals can be added to a non-supersaturated solution in order to provide a site for material crystallization. It is well known to those skilled in the art that following the completion of a product-generation reaction, seed crystals may be added to the non-supersaturated product solution followed by, for example, pH adjustment or cooling at a predetermined rate in order to generate and maintain a sufficient level of supersaturation, thus driving force for product crystallization. In those methods supersaturation is induced, for example, by solubility reduction via pH adjustment or cooling, but not dynamically by product generation via chemical reaction. See generally U.S. Pat. No. 5,047,088 to Liaw. The prior art does not describe seeding dynamic systems wherein product generation and crystallization occur simultaneously.
One of skill in the art will also recognize that rapid induction of a high degree of supersaturation in the absence of seed crystals can result in instantaneous, uncontrolled, nucleation and crystallization with limited post-nucleation crystal growth. Such crystals are typically small with high impurity inclusion. Generally, generation of high supersaturation is done by instantaneously reducing solubility, for example by pH adjustment to the solute isoelectric point, addition of a solvent in which the solute exhibits low solubility, or rapid temperature reduction by “flash crystallization.”
The prior art provides little teaching regarding controlling particle size and particle size distribution in dynamic processes in which product generation and crystallization from solution occur concurrently. In such systems supersaturation is induced by reaction rather than by solubility reduction through, for example, pH adjustment or temperature change. Likewise, little guidance is provided in the art for controlling crystal growth rate in such dynamic systems.
U.S. Pat. Nos. 5,011,988 and 5,338,530 to Thunberg describe the crystallization of iminotriacetic acid (N(CH2COOH)3 or “NTA”) from a mother liquor solution containing about 2.9% NTA, 6.3% IDA and 22.6% Na2SO4. The mother liquor pH is adjusted to the NTA isoelectric point of about 2.1 followed by NTA seeding to initiate crystallization. Notably the initial concentration of NTA is low; thus, high degrees of supersaturation are never achieved.
Although PMIDA processes known in the art are of significant utility, a need persists for improvements that might further reduce product cost, increase product purity and throughput, and reduce environmental impact. A significant advancement over the prior art could be achieved by developing a high throughput process whereby PMIDA crystallization is controlled to yield product characterized by crystals having a low degree of impurity inclusion, and having a large, uniform and porous crystalline morphology. Moreover, the crystals should be capable of forming a uniform, substantially porous bed in solid-liquid separation equipment and exhibit a high dewatering rate thereby maximizing the effectiveness of removing impurities by washing, and material throughput. It would be a further advancement if the crystals could be quickly prepared in a controlled manner by, for example, crashing from solution or flash crystallization thereby obviating the need for protracted rate controlled crystallization cooling cycles.